Spinal fixation constructs are utilized to provide stability to the spine. Most often the fixation construct is used as an adjunct to fusion surgery during which adjacent vertebrae are prepared to facilitate bone growth between them, thereby eliminating motion between the vertebrae. Because motion between the vertebrae tends to inhibit bone growth, the fixation constructs are employed to prevent motion so that bone can grow and achieve a solid fusion. When the position of one or more vertebrae must be adjusted to restore a more natural alignment of the spinal column, the fixation construct also serves to maintain the new alignment until fusion is achieved. Fixation constructs of various forms are well known in the art. Most commonly, the fixation construct is a plate anchored to the anterior column with multiple bone anchors or a posterior fixation construct including multiple anchors and a connecting rod anchored to the posterior elements of the spine. For a posterior fixation construct the anchors (typically pedicle screws) are anchored into the pedicles of each vertebra of the target motion segment. The anchors are then connected by a fixation rod that is locked to each anchor, thus eliminating motion between the adjacent vertebrae of the motion segment. The posterior fixation construct may be applied unilaterally or bilaterally. Additionally the posterior fixation construct may be applied across multiple levels or motion segments.
The fixation anchors utilized in posterior fixation constructs generally include an anchor portion and a rod housing. The rod housing includes a pair of upstanding arms separated by a rod channel in which the fixation rod is captured and locked. When constructing the posterior fixation construct the surgeon must align and seat the rod in the rod channel. This can be a challenge, particularly when one or more of the vertebrae to be connected is out of alignment leaving the associated anchor offset vertically and/or horizontally from the remaining anchor(s) of the construct. Constructing the posterior fixation construct under minimally invasive access conditions (e g minimizing overall incision length and muscle stripping as compared to traditional open procedures) also increases the difficulty of aligning the rod with the rod channel of the anchor.
Rod reducers are often used to facilitate alignment and seating of the rod in the anchor housing. Rod reducers often work by engaging the anchor housing with a first portion and then driving the rod towards the housing and/or pulling the housing towards the rod with a second portion that moves relative to the first portion. With minimally invasive systems the reducer may attach to a guide assembly which is itself attached to the anchor. Generally, reducers employed with minimally invasive systems must have a portion that passes over and translates along the outside of the guide to engage the rod, or, pass directly through the center of the guide. While both of these configurations may be effective, there are disadvantages. For example, passing a reducer over the outside of the guide requires increasing the incision size in order to fit the reducer. On the other hand, passing the reducer through the center of the guide provides for one point of contact on the rod, in the middle of the anchor housing, which leaves a potential for the rod not to align fully with the housing. The instruments, tools, and techniques described herein are directed towards reducing these challenges and others associated with posterior spinal fixation.